The Electronic Module Test (EMT) has been widely used to detect latent defects in high density multilayer substrates and circuit boards. Versions of EMT testers are disclosed in U.S. Pat. Nos. 4,496,900 and 4,868,506 assigned to the assignee of the present invention.
FIG. 1 illustrates a typical test set up for an EMT tester 10. A test instrument 12 is connected to a substrate 14, placed in a substrate holder 16, by a pair of conductors 18 and 19. The conductors are connected to electrically conductive test probes 20 and 22, which may be physically moved into contact with the substrate 14 to contact conductors thereon.
These probes are positioned at high speed by positioning devices such as those disclosed in U.S. Pat. Nos. 5,180,955, 5,291,112 and 5,153,472. Such positioning mechanisms rapidly move the probes into place, after which a test is conducted by test instrument 12. The positioning devices then rapidly move the probes 20 and 22 to a next set of positions to conduct a test on different conductors of electronic module 14.
Test instrument 12 includes system control unit 24, which may be microprocessor, or a suitably programmed personal computer. System control unit 24 controls a test signal generator 26 which provides a signal to probes 20 and 22 by way of conductors 18 and 19. Also connected to conductors 18 and 19 is a measurement circuit 28 including appropriate preamplifiers for acquiring signals from substrate 14 during a test. After suitable amplification, these signals are sent to a signal processing unit 30 which also operates under the control of system control unit 24. An output is eventually provided by signal processing unit 30 indicating whether a test has been passed or failed.
Testers of the type described above must detect a small fixed-frequency signal generated by defects in the substrate 14. Since the signal is of low amplitude, it must not only be amplified, but it must be filtered to remove noise and transients. Generally, conventional analog filters were used. However, there are two major problems using such conventional analog filters. One is the slow rise time of the filter. The other is that the transient signal due to probe bounce is generally large in amplitude and blocks the signal. It is necessary to wait a significant amount of time before the transient dies out and the filtered signal is suitable as the basis for making a determination as to whether the device under test has a defect.
One way to block the transient signal into the analog filter is to wait a fixed period of time, which is usually estimated by using the worst case of probe bounce experienced after the test probes have been placed on the substrate 14 being tested. This approach adds an undue wait time for every measurement of the many thousands of measurements that may be conducted on the substrate 14. This delay unduly reduces the possible through-put for the tester 10.